What Is Ohm's Law and How to Use It

Ohm's law describes how voltage, current, and resistance relate in an electrical circuit: voltage equals current times resistance, written V = I × R. Voltage (volts) is the electrical push, current (amps) is the flow of charge, and resistance (ohms) is how much the circuit opposes that flow.

Rearranging the same equation lets you find any of the three from the other two: current is I = V / R, and resistance is R = V / I. It's the single most-used formula in electronics.

Electrical power, measured in watts, is how fast energy is used, and it ties into Ohm's law through P = V × I. Combine that with V = I × R and you get two more forms: P = I²R and P = V²/R.

Together these four quantities — voltage, current, resistance, and power — form a set where knowing any two lets you calculate the other two. That's exactly what the calculator does.

Say you're driving an LED that needs 20 milliamps (0.02 A) from a 5-volt supply, and the LED drops 2 volts, leaving 3 volts across the resistor. Ohm's law gives R = V / I = 3 / 0.02 = 150 ohms.

You can also check the power the resistor dissipates: P = I²R = 0.02² × 150 = 0.06 watts, comfortably within a standard quarter-watt resistor. Enter any two known values and the calculator handles the rest.